As more organisms developed hard
bodies, and single celled organisms
that absorbed and concentrated
calcium multiplied, ocean floors
became increasingly rich in lime
deposits, or lime mud. This is the
beginning of limestone. Subsequent
layers buried previous ones, pressing
the lower layers into rock.
Things were going along very nicely
when suddenly North America
crashed/drifted into another land
mass. Around 300 million years ago
the world's land masses reunited into
another super continent called
Pangea. But like the last super
continent, this one would eventually
break up. About 180 million years ago
North America and South America
separated or rifted creating the
Atlantic Ocean. North America and
Europe continue to spread apart,
widening the Atlantic every year. 180
million years ago the continents were
separating at a rapid 5 cm./yr., an
accident was imminent.